Refinery economics encourage full utilization of a broad spectrum of feedstocks for lubricant refining even though the lower quality feeds are more difficult, if not impossible, to process into quality products. Blending materials of higher quality with materials of lower quality to produce a lubricant meeting predetermined specifications accomplishes full use of a more complete range of products.
The quality of a hydrocarbon feedstock destined for lubricant refining impacts the final lubricant properties including the boiling point, viscosity, viscosity index (VI), pour point and other properties.
The pour point is the lowest temperature at which the stock will flow, while the measurement of viscosity indicates the lubricant's resistance to flow which tends to thin or decrease as the temperature increases and thicken or increase as the temperature decreases. The amount of thinning is critical to lubricant performance because it impacts the ability to lubricate successfully at high temperatures. Thus, it is important to know how fast the lubricant decreases in viscosity as the temperature goes up and the Viscosity Index (VI) is a measurement of this effect. There is a trend towards more severe service ratings as engines become more efficient leading to higher engine temperatures. This requires higher V. I. to ensure that the lubricants will have adequate viscosity at high temperatures without excessive viscosity at lower temperatures. In part, these essential properties may be improved by additives but advances are needed in basestock performance to accommodate more severe service requirements.
The feedstock dictates the choice of refining process since all refining processes are not suitable for all types of feedstocks. Although many lubricant refining processes have been proposed for preparing lubricants and improving their properties; in general, lower quality feedstocks require more severe treatment to produce a suitable product meeting the minimum service specifications. Since refinery economics often dictate using a lower quality feedstock for lubricant refining, there is a need for processes which enable the refiner to obtain higher quality lubricants from low quality lubricant feedstocks under less severe, and less costly, process conditions. Low severity process conditions translate to a more valuable commodity at a lower cost.
The starting point for producing mineral oil lubricants is in the atmospheric or vacuum distillation tower. Distillation separates the crude oil into different components by their boiling range. The lubricant boiling range fraction, which boils above about 650.degree. F., makes the charge stock for lubricant refining. The components of the lubricant charge stock include paraffins, naphthenes, aromatics, resins and asphaltenes. The paraffinic and naphthenic distillate fractions are generally referred to as the neutrals, e.g. heavy neutral and light neutral. Although the heavy neutral is characterized by a higher percentage of naphthenes and the light neutral is characterized by a higher percentage paraffins, both contain some aromatics along with some paraffins and naphthenes. Because the aromatic components lead to high viscosity and extremely poor viscosity indices, highly aromatic asphaltic type crudes are not the preferred feedstocks. The resins and alphaltenes are undesirable because they are too viscous and contain high levels of metals and sulfur. The paraffinic and naphthenic crude stocks are preferred yet their lubricant qualities conflict. The more paraffinic stocks make good lubricants because they possess excellent viscosity properties, yet the long straight chain paraffinic component encourages an undesirably high pour point. On the other hand, the naphthenic stocks have the desirable low pour point but have poor viscosity properties.
To produce an effective high performance lubricant, the differences between the lower pour point more paraffinic stocks and the deficient viscosity properties of the aromatic and naphthenic stocks must be reconciled. This is achieved by subjecting the feedstock to various refining processes which physically separate the undesirable components and/or chemically convert the undesirable components to more desirable components.
Since aromatics are present to varying degrees in paraffinic and naphthenic stocks, their removal is necessary to obtain optimum lubricant properties. The aromatics are extracted by solvent extraction using a solvent such as phenol, furfural, N-methylpyrolidone or another material which is selective for the extraction of the aromatic components. After solvent extraction, the paraffinic stocks are usually subjected to a dewaxing step to remove the waxy paraffinic components which contribute to the high pour point. The dewaxing step is usually the last major step in the lubricant refining process.
A number of dewaxing processes are known in the petroleum refining industry. One of these, solvent dewaxing with solvents such as methylethylketone (MEK) and liquid propane, has achieved the widest use in the industry. MEK is a solvent for the naphthenic component and the highly branched paraffinic component which has adequate pour point properties. Solvent dewaxing leaves behind the high pour point waxy component (the long straight chain paraffins) in the form of a solid which can be filtered. This solid is called slack wax.
Recently, catalytic dewaxing processes have been proposed for the production of lubricating oil stocks and these processes possess a number of advantages over the conventional solvent dewaxing procedures. The catalytic dewaxing processes are generally similar to processes for dewaxing the middle distillate fractions such as heating oils, jet fuels and kerosenes. The Mobil Lube Oil Dewaxing Process (MLDW) has now reached maturity and is capable of producing low pour point oils not attainable by solvent dewaxing.
Generally, the catalytic dewaxing processes operate by selectively cracking the longer chain end paraffins to produce lower molecular weight products which may then be removed by distillation from the higher boiling lubricant stock. The catalysts which have been proposed for this purpose have usually been zeolites which have a pore size which admits the straight chain, waxy n-paraffins either alone or with only slightly branched chain paraffins but which exclude more highly branched materials and cycloaliphatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, and the synthetic ferrierites ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes, as described in U.S. Pat. Nos. 3,894,938, 4,176,050, 4,181,598, 4,222,855, 4,229,282 and 4,247,388. A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174. The relationship between zeolite structure and dewaxing properties is discussed in J. Catalysis 86, 24-31 (1984).
The conventional catalytic dewaxing processes using intermediate pore size zeolites, such as ZSM-5 can cause a yield loss since the components which are in the desired boiling range undergo a bulk conversion to lower boiling fractions. Although the lower boiling fractions may be useful in other products, they must be removed from the lubricant stock. A notable advance in the dewaxing process is described in U.S. Pat. Nos. 4,419,220 and 4,518,485, in which the waxy components of the feed, comprising straight chain and slightly branched chain paraffins, are converted by isomerization over a catalyst based on zeolite beta and a hydrogenation-dehydrogenation component which is typically a base metal or a noble metal, usually of group VIA or VIIIA of the Periodic Table of the Elements such as cobalt, molybdenum, nickel, tungsten, palladium or platinum. During the isomerization, the waxy components are converted to relatively less waxy isoparaffins and at the same time, the slightly branched chain paraffins undergo isomerization to more highly branched aliphatics. Some cracking does take place so that not only is the pour point reduced by reason of the isomerization but, in addition, the heavy ends undergo some cracking or hydrocracking to form liquid range materials which contribute to a low viscosity product. As described in U.S. Pat. No. 4,518,485, the isomerization dewaxing step may be proceeded by a hydrotreating step in order to remove heteroatom-containing impurities, which may be separated in an interstage separation process similar to that employed in two-stage hydrotreating-hydrocracking processes.
Although the catalytic dewaxing processes are commercially important because they do not produce quantities of solid paraffin wax which is a less desirable, low value product, some have certain disadvantages. Because of the disadvantages, combining the catalytic dewaxing processes with other processes in order to produce lube stocks of satisfactory properties has been proposed. For example, U.S. Pat. No. 4,181,598 discloses a method for producing a high quality lubricant base stock by subjecting a waxy fraction to solvent refining, followed by catalytic dewaxing with subsequent hydrotreatment of the product. U.S. Pat. No. 4,428,819 discloses a process for improving the quality of catalytically dewaxed lubricant stocks by subjecting the catalytically dewaxed oil to a hydroisomerization process which removes residual quantities of petrolatum wax which contribute to poor performance in the Overnight Cloud Point test (ASTM D2500-66).
The use of peroxide treatment for modifying the viscosity of various lubestocks including distillates and hydrocracked resids has been described in U.S. Pat. Nos. 3,128,246 and 3,594,320. Other peroxide treatment processes are described in U.S. Pat. Nos. 4,594,172 and 4,618,737. Peroxide treatment has been suggested for coupling or dimerization of Fischer-Tropsch paraffins. In U.S. Patent No. 4,594,172 a synthesis gas conversion process is described in which the synthesis gas is converted to paraffins by Fischer-Tropsch. The resulting C.sub.10 to C.sub.19 fraction is given two peroxide treatments to couple the paraffins and produce a C.sub.20.sup.+ wax fraction.
In U.S. Patent No. 5,037,528 there is described a process for making high VI lubricant base stocks by the peroxide-promoted oligomerization of wax-derived lubricant fractions, such as a slack wax or deoiled wax. Because the increase in viscosity is related to the amount of peroxide used with greater viscosity increases resulting from greater amounts of peroxide, the peroxide requirements for this process depend on the desired increase in viscosity. The peroxide requirements are on the order of about 1 to 50, preferably from 4 to 20 weight percent of the oil. This relationship, between the proportion of peroxide used and the viscosity increase, is essentially exponential both in the batch and continuous reaction. The process is exceedingly efficient in its use of peroxide; that is, large amounts of peroxide are unnecessary to achieve efficient coupling of the paraffinic components to produce the higher molecular weight oligomers. The described coupling can be achieved by the use of 10 o 20 wt. % peroxide as opposed to 100 weight % peroxide described in U.S. Pat. No. 4,594,172 which is a significant economic savings. U.S. Pat. No. 5,037,528 is incorporated herein by reference in its entirety.
Lubricant blending is one of the last steps in the manufacture of a lubricant and is employed to meet viscosity and VI requirements. Lubricant blending comprises mixing different materials boiling in the lubricant boiling range to make a final homogenous product. Usually a higher viscosity lubricant will be blended with a lower viscosity lubricant to achieve a blended lubricant of intermediate viscosity. The viscosity properties of the blended lubricant can usually be predicted by a blending relationship which has been accepted as a standard by the American Society for Testing and Materials (ASTM) designation D 341-87.
The ASTM D 341-87 adopted kinematic viscosity-temperature charts as a convenient method for ascertaining the kinematic viscosity of a liquid hydrocarbon at any temperature within a range, provided that kinematic viscosities at two temperatures are known. The charts are based on the following mathematical relationship: EQU log log Z=A-B logT
wherein
Z=(v+0.7+C-D+E-F+G-H); PA1 v= kinematic viscosity, cSt.; PA1 T= temperature, K or .degree.R; PA1 A= log log Z.sub.B(40) ; PA1 B= log log Z.sub.B(100) ; PA1 C= log log Z.sub.L(40) ; PA1 D= log log Z.sub.L(100) ; PA1 E= log log Z.sub.H(40) ; PA1 F= log log Z.sub.H(100) ; PA1 B= blend; PA1 L= low viscosity oil; PA1 H= high viscosity oil; PA1 (40)=40.degree. C.; and PA1 (100)=100.degree. C. PA1 V.sub.1 and V.sub.2 are the volumes of the oils making up the blend; and PA1 V is the volume of the blend.
The viscosity characteristics of lubricant blends based on predictions or estimations arrived at from the ASTM method are usually relied on in evaluating whether a particular blend will meet the necessary kinematic viscosity and VI specifications or the target kinematic viscosity and VI. This blending relationship between oils is further described in appendixes Xl amd X2 of ASTM D 341-87. Although the ASTM method is not the only available test for approximating the viscosity of a blend of oils, it is a widely accepted standard. From the calculation of the viscosity according to the ASTM method, the VI can be calculated using the ASTM D2270-86 standard method for calculating viscosity index. The actual VI is also calculated from the ASTM D2270-86 method using the actual kinematic viscosities of the lubricant measured at 40.degree. C. and 100.degree. C.
Another lubricant blending rule is the Arrhenius Rule where: ##EQU1## where: n.sub.o is the viscosity of the blend, n.sub.1 and n.sub.2 are the viscosities of the component oils;
Although the Arrhenius Rule was established for dynamic viscosities, it can be extended to cover kinematic viscosities.